rfc 5137 requires 4...6 hexdigits

`U+0041` is mistakenly represented as `\u'41'`. It should be `\u'0041'` instead, see [rfc 5137](http://tools.ietf.org/html/rfc5137#section-5.1):
```
EmbeddedUnicodeChar =  %x5C.75.27 4*6HEXDIG %x27
      ; starting with lowercase "\u" and "'" and ending with "'".
```